Green’s Dictionary of Slang

1. (US) the truth.

1935

A.J. Pollock Und. Speaks 36/2: Ephus, dependable information; the low down.

2. a trick or gimmick.

1958		L.F. Cooley Run For Home (1959) 7: There’s something about those Russians. They got the ephus on everybody. I don’t know what it is. They cut and bleed and die just like everyone else, but they’ve got everybody believing they’re tough as shark hide.
1959		C. Lederer Never Steal Anything Small (film): The gimmick, the gizmo, the gadget, the ephus. If I could just think of the ephus [HDAS].